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Three previously defined antigenic subsets of C3 (C3(S), C3(D) alpha and C3(D)beta) clearly distinguish native C3 from C3 that has been denatured in the presence of sodium dodecyl sulphate (C3-SDS): C3-SDS expresses antigens of all 3 subsets, while native C3 only expressed C3(S) antigens. The radioimmunometric assessment performed in this study revealed that 90% of the C3(S) and C3(D)alpha and 70% of the C3(D)beta antigens demonstrable in C3-SDS also was expressed by physiologically bound C3b on sheep erythrocytes. The antigenic properties of bound C3b were not shared by soluble C3b, which (like soluble C3) only expressed C3(S) antigens, nor did soluble C3b 'spontaneously' released from complement-treated target cells express antigens other than C3(S). We therefore conclude that the expression of C3(D) antigens, under physiological conditions of activation, reflects a conformational modification unique for bound C3b and occurring to an extent comparable to that of C3 in the presence of a strong denaturant. Additional studies further revealed that the antigenic profile of bound C3b is determined by a heterogeneous population of molecules differing in their relative expression of the C3(S), C3(D)alpha and C3(D)beta subsets.